.\" This is -*-nroff-*-
.\" XXX standard disclaimer belongs here....
.\" $Header: /usr/local/cvsroot/postgres95/src/man/large_objects.3,v 1.3 1996/12/11 00:27:51 momjian Exp $
.TH "LARGE OBJECTS" INTRO 03/18/94 PostgreSQL PostgreSQL
.SH DESCRIPTION
.PP
In Postgres, data values are stored in tuples and individual tuples
cannot span data pages. Since the size of a data page is 8192 bytes,
the upper limit on the size of a data value is relatively low. To
support the storage of larger atomic values, Postgres provides a large
object interface.  This interface provides file-oriented access to
user data that has been declared to be a large type.
.PP
This section describes the implementation and the
programmatic and query language interfaces to Postgres large object data.
.PP
.SH "Historical Note"
.PP
Originally, postgres 4.2 supports three standard implementations of large
objects: as files external to Postgres, as Unix files managed by Postgres, and as
data stored within the Postgres database. It causes considerable confusion
among users. As a result, we only support large objects as data stored 
within the Postgres database in Postgres. Even though is is slower to access,
it provides stricter data integrity and time travel. For historical reasons,
they are called Inversion large objects. (We will use Inversion and large
objects interchangeably to mean the same thing in this section.)
.SH "Inversion Large Objects"
.PP
The Inversion large
object implementation breaks large objects up into \*(lqchunks\*(rq and
stores the chunks in tuples in the database.  A B-tree index
guarantees fast searches for the correct chunk number when doing
random access reads and writes.
.SH "Large Object Interfaces"
.PP
The facilities Postgres provides to access large objects, both in
the backend as part of user-defined functions or the front end
as part of an application using the \*(LQ interface, are described 
below. (For users familiar with postgres 4.2, Postgres has a new set of 
functions providing a more coherent interface. The interface is the same
for dynamically-loaded C functions as well as for \*(LQ.
.PP
The Postgres large object interface is modeled after the Unix file
system interface, with analogues of
.I open (2),
.I read (2),
.I write (2),
.I lseek (2),
etc.  User functions call these routines to retrieve only the data of
interest from a large object.  For example, if a large object type
called
.I mugshot
existed that stored photographs of faces, then a function called
.I beard
could be declared on
.I mugshot
data.
.I Beard
could look at the lower third of a photograph, and determine the color
of the beard that appeared there, if any.  The entire large object
value need not be buffered, or even examined, by the
.I beard
function.  
.\"As mentioned above, Postgres supports functional indices on
.\"large object data.  In this example, the results of the
.\".I beard
.\"function could be stored in a B-tree index to provide fast searches
.\"for people with red beards.
.PP
Large objects may be accessed from dynamically-loaded C functions
or database client programs that link the Libpq library.
Postgres provides a set of routines that
support opening, reading, writing, closing, and seeking on large
objects.  
.SH "Creating a Large Object"
.PP
The routine
.nf
Oid lo_creat(PGconn *conn, int mode)
.fi
creates a new large object. The 
.I mode
is a bitmask describing several different attributes of the new
object.  The symbolic constants listed here are defined in
.nf
/usr/local/postgres95/src/backend/libpq/libpq-fs.h
.fi
The access type (read, write, or both) is controlled by
.SM OR
ing together the bits
.SM INV_READ
and
.SM INV_WRITE .
If the large object should be archived \(em that is, if
historical versions of it should be moved periodically to a special
archive relation \(em then the
.SM INV_ARCHIVE
bit should be set.  The low-order sixteen bits of
.I mask
are the storage manager number on which the large object should
reside.  For sites other than Berkeley, these bits should always be
zero.  
.\"At Berkeley, storage manager zero is magnetic disk, storage
.\"manager one is a Sony optical disk jukebox, and storage manager two is
.\"main memory.
.PP
The commands below create an (Inversion) large object:
.nf
inv_oid = lo_creat(INV_READ|INV_WRITE|INV_ARCHIVE);
.fi
.SH "Importing a Large Object"
To import a UNIX file as a large object, call
.nf
Oid
lo_import(PGconn *conn, text *filename)
.fi
The 
.I filename
argument specifies the UNIX pathname of the file to be imported as
a large object.
.SH "Exporting a Large Object"
To export a large object into UNIX file, call
.nf
int
lo_export(PGconn *conn, Oid lobjId, text *filename)
.fi
The 
.I lobjId
argument specifies the Oid of the large object to export and 
the 
.I filename
argument specifies the UNIX pathname of the file.
.SH "Opening an Existing Large Object"
.PP
To open an existing large object, call
.nf
int
lo_open(PGconn *conn, Oid lobjId, int mode, ...)
.fi
The
.I lobjId
argument specifies the Oid of the large object to open.
The mode bits control whether the object is opened for reading
.SM INV_READ ), (
writing
.SM INV_WRITE ), (
or both.  
.PP
A large object cannot be opened before it is created.
.B lo_open
returns a large object descriptor for later use in 
.B lo_read ,
.B lo_write ,
.B lo_lseek ,
.B lo_tell ,
and
.B lo_close .
.\"-----------
.SH "Writing Data to a Large Object"
.PP
The routine
.nf
int
lo_write(PGconn *conn, int fd, char *buf, int len)
.fi
writes
.I len
bytes from
.I buf
to large object
.I fd .
The
.I fd
argument must have been returned by a previous
.I lo_open .
.PP
The number of bytes actually written is returned.
In the event of an error,
the return value is negative.
.SH "Seeking on a Large Object"
.PP
To change the current read or write location on a large object,
call
.nf
int
lo_lseek(PGconn *conn, int fd, int offset, int whence)
.fi
This routine moves the current location pointer for the large object
described by
.I fd
to the new location specified by
.I offset .
The valid values for .I whence are
.SM SEEK_SET
.SM SEEK_CUR
and
.SM SEEK_END.
.\"-----------
.SH "Closing a Large Object Descriptor"
.PP
A large object may be closed by calling
.nf
int
lo_close(PGconn *conn, int fd)
.fi
where
.I fd
is a large object descriptor returned by
.I lo_open .
On success,
.I lo_close
returns zero.  On error, the return value is negative.
.PP
.SH "Built in registered functions"
.PP
There are two built-in registered functions,
.I lo_import
and
.I lo_export
which are convenient for use in SQL queries.   
.PP
Here is an example of there use
.nf
CREATE TABLE image (
        name            text,
        raster          oid
);

INSERT INTO image (name, raster)
   VALUES ('beautiful image', lo_import('/etc/motd'));

SELECT lo_export(image.raster, '/tmp/motd') from image
   WHERE name = 'beautiful image';
.fi
.PP
.SH "Accessing Large Objects from LIBPQ"
Below is a sample program which shows how the large object interface in
\*(LP can be used.  Parts of the program are commented out but are left
in the source for the readers benefit.  This program can be found in 
.nf
\&../src/test/examples
.fi
.PP
Frontend applications which use the large object interface in \*(LP
should include the header file
.B "libpq/libpq-fs.h"
and link with the
.B libpq
library.
.bp
.SH "Sample Program"
.nf
/*-------------------------------------------------------------------------
 *
 * testlo.c--
 *    test using large objects with libpq
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    $Header: /usr/local/cvsroot/postgres95/src/man/large_objects.3,v 1.3 1996/12/11 00:27:51 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include <stdio.h>
#include "libpq-fe.h"
#include "libpq/libpq-fs.h"

#define BUFSIZE		1024

/*
 * importFile -
 *    import file "in_filename" into database as large object "lobjOid"
 *
 */
Oid importFile(PGconn *conn, char *filename)
{
    Oid lobjId;
    int lobj_fd;
    char buf[BUFSIZE];
    int nbytes, tmp;
    int fd;

    /*
     * open the file to be read in
     */
    fd = open(filename, O_RDONLY, 0666);
    if (fd < 0)  {   /* error */
	fprintf(stderr, "can't open unix file\\"%s\\"\\n", filename);
    }

    /*
     * create the large object
     */
    lobjId = lo_creat(conn, INV_READ|INV_WRITE);
    if (lobjId == 0) {
	fprintf(stderr, "can't create large object");
    }
    
    lobj_fd = lo_open(conn, lobjId, INV_WRITE);
    /*
     * read in from the Unix file and write to the inversion file
     */
    while ((nbytes = read(fd, buf, BUFSIZE)) > 0) {
	tmp = lo_write(conn, lobj_fd, buf, nbytes);
	if (tmp < nbytes) {
	    fprintf(stderr, "error while reading \\"%s\\"", filename);
	}
    }
    
    (void) close(fd);
    (void) lo_close(conn, lobj_fd);

    return lobjId;
}

void pickout(PGconn *conn, Oid lobjId, int start, int len)
{
    int lobj_fd;
    char* buf;
    int nbytes;
    int nread;

    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd < 0) {
	fprintf(stderr,"can't open large object %d",
		lobjId);
    }

    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len+1);
    
    nread = 0;
    while (len - nread > 0) {
	nbytes = lo_read(conn, lobj_fd, buf, len - nread);
	buf[nbytes] = '\\0';
	fprintf(stderr,">>> %s", buf);
	nread += nbytes;
    }
    fprintf(stderr,"\\n");
    lo_close(conn, lobj_fd);
}

void overwrite(PGconn *conn, Oid lobjId, int start, int len)
{
    int lobj_fd;
    char* buf;
    int nbytes;
    int nwritten;
    int i;

    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd < 0) {
	fprintf(stderr,"can't open large object %d",
		lobjId);
    }

    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len+1);
    
    for (i=0;i<len;i++)
	buf[i] = 'X';
    buf[i] = '\\0';

    nwritten = 0;
    while (len - nwritten > 0) {
	nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
	nwritten += nbytes;
    }
    fprintf(stderr,"\\n");
    lo_close(conn, lobj_fd);
}


/*
 * exportFile -
 *    export large object "lobjOid" to file "out_filename"
 *
 */
void exportFile(PGconn *conn, Oid lobjId, char *filename)
{
    int lobj_fd;
    char buf[BUFSIZE];
    int nbytes, tmp;
    int fd;

    /*
     * create an inversion "object"
     */
    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd < 0) {
	fprintf(stderr,"can't open large object %d",
		lobjId);
    }

    /*
     * open the file to be written to
     */
    fd = open(filename, O_CREAT|O_WRONLY, 0666);
    if (fd < 0)  {   /* error */
	fprintf(stderr, "can't open unix file\\"%s\\"",
		filename);
    }

    /*
     * read in from the Unix file and write to the inversion file
     */
    while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) > 0) {
	tmp = write(fd, buf, nbytes);
        if (tmp < nbytes) {
	    fprintf(stderr,"error while writing \\"%s\\"",
		    filename);
	}
    }

    (void) lo_close(conn, lobj_fd);
    (void) close(fd);

    return;
}

void 
exit_nicely(PGconn* conn)
{
  PQfinish(conn);
  exit(1);
}

int
main(int argc, char **argv)
{
    char *in_filename, *out_filename;
    char *database;
    Oid lobjOid;
    PGconn *conn;
    PGresult *res;

    if (argc != 4) {
	fprintf(stderr, "Usage: %s database_name in_filename out_filename\\n",
		argv[0]);
	exit(1);
    }

    database = argv[1];
    in_filename = argv[2];
    out_filename = argv[3];

    /*
     * set up the connection
     */
    conn = PQsetdb(NULL, NULL, NULL, NULL, database);

    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) == CONNECTION_BAD) {
	fprintf(stderr,"Connection to database '%s' failed.\\n", database);
	fprintf(stderr,"%s",PQerrorMessage(conn));
	exit_nicely(conn);
    }
	
    res = PQexec(conn, "begin");
    PQclear(res);
    printf("importing file \\"%s\\" ...\\n", in_filename);
/*  lobjOid = importFile(conn, in_filename); */
    lobjOid = lo_import(conn, in_filename); 
/*
    printf("\\tas large object %d.\\n", lobjOid);

    printf("picking out bytes 1000-2000 of the large object\\n");
    pickout(conn, lobjOid, 1000, 1000);

    printf("overwriting bytes 1000-2000 of the large object with X's\\n");
    overwrite(conn, lobjOid, 1000, 1000);
*/

    printf("exporting large object to file \\"%s\\" ...\\n", out_filename);
/*    exportFile(conn, lobjOid, out_filename); */
    lo_export(conn, lobjOid,out_filename);

    res = PQexec(conn, "end");
    PQclear(res);
    PQfinish(conn);
    exit(0);
}
.fi
